i got my fish a little less than a month ago. he had some black edging on his tail and his tail seemed to have a cut in between, im a newbie and assumed he was half double tail. anyway hes seemed fine. today i look into his tank and omg his tail has dark black edging is completely ragged and smaller than it used to be I THINK HE HAS FIN ROT...i ran to the petstore bought aquarium salt i have stress coat. hes in a tank that is around 1 gallon i think, what dosage am i supposed to give (i dont have a teaspoon..how the heck do i measure this) please help me im a newbie and im toooootally panicking right now
THANKS

omg..i just looked at my other 5 fish to make sure they were all ok..realized another one also has finrot..im about to cry..how did i not detect this earlier...this guy is in a 5 gallon..what do i do please help

and don't get me wrong, im a good fish keeper, i do water changes as often as i need to based on their tanks (the ones in 1 gallons are only temporary, the new 5 galls havent arrived yet in the mail) and i feed them 3 pellets every day and htey have nice tanks with plants and caves and stuff..i try to be responsible and hten this happens :(

Housing
What size is your tank? 1 gal (its only a temporary, he's been in it for about two weeks)
What temperature is your tank? anywhere from 76-81 degrees, a couple degrees chillier at night (i have no heater but i have a thermometer. i live in the tropics so its always hot here and i moniter the temp, air conditioner is usually off)
Does your tank have a filter? nope
Does your tank have an air stone or other type of aeration? nope
Is your tank heated? nope
What tank mates does your betta fish live with? none

Food
What type of food do you feed your betta fish? pellets and bloodworms
How often do you feed your betta fish? every day 3 bellets, or 2 pellets in the morning and 1 bloodworm at night

Maintenance
How often do you perform a water change? 3 times per week..is this too many or too few? i know that with finrot it needs to be 100% every day
What percentage of the water do you change when you perform a water change? 100%
What type of additives do you add to the water when you perform a water change? stress coat

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Symptoms and Treatment
How has your betta fish's appearance changed? as described above, tail is jagged with black outlining
How has your betta fish's behavior changed? hasn't changed, hes active and loves eating
When did you start noticing the symptoms? when i first got him i noticed the black edging and a slight indentation, i dismissed it. 30 minutes ago i noticed the symptom described above
Have you started treating your fish? If so, how? i havent, im setting up the tank now, i put stress coat but havent put aq salt in yet
Does your fish have any history of being ill? no
How old is your fish (approximately)? id say like 6 months maybe less

like i said earlier, it appears to me that two of my fish have fin rot. i had the orange guy in a 5 gallon, but i moved him to a 1 gallon and put a blue guy in the 5 gallon..i did not perform a water change, teh water was clean so i just put him in the same water. could the water have been contaminated ?